Inactivation or loss of p53 is a common event associated with the development of human cancers. Functional inactivation may occur as a consequence of genetic aberrations within the p53 gene, most commonly missense mutations, or interaction with vital and cellular oncogenes [for reviews see: Levine, A. J. et al, Nature 351,453-455 (1991), Vogelstein, B. and Kinzler, K. W., Cell 70, 523-526 (1992), Zambetti, G., and Levine, A. J., FASEB J. 7, 855-865 (1993), Harris, C. C., Science 262, 1980-1981 (1993)]. Loss of wild-type (wt) p53 functions leads to uncontrolled cell cycling and replication, inefficient DNA repair, selective growth advantage and, consequently, tumor formation [Levine, A. J. et al, Nature 351, 453-455 (1991 ); Vogelstein, B. and Kinzler, K. W., Cell 70, 523-526 (1992); Zambetti G. and Levine, A. J., FASEB J. 7, 855-865 (1993); Harris, C. C., Science 262, 1980-1981, (1993); Lane, D. P., Nature 358, 15-16 (1992); Livingstone, L. R. et al, Cell 70, 923-935 (1992)]. Tumorigenesis may be even further accentuated by the gain of new functions associated with many mutant forms of p53 [Chen, P.-L. et al, Science 250, 1576-1580 (1990); Dittmer, D. et al, Nature Genetics 4, 4142-4145 (1993); Sun, Y. et al, Proc. Natl Acad. Sci. U.S.A. 90, 2827-283 1 (1993)], providing a potential basis for their strong selection in human tumors.
The mechanism(s) underlying p53 mediated growth suppression in still ill defined. However, of particular interest is the ability of p53 to act as a transcription factor, a function which strongly correlates with its ability to act as a tumor suppressor. p53 has been shown to suppress a variety of promoters containing TATA elements [e.g. Ginsberg, D. et al, Proc. Natl. Acad. Sci. U.S.A. 88, 9979-9983 (1993), Santhanam, U. et al, Proc. Natl. Acad. Sci. U.S.A. 88, 7605-7609 (1993), Kley, N. et al, Nucleic Acids Res. 20, 4083-4087 (1992), Mack, D. H. et al, Nature 363, 281-283 (1993)]. This suppression is sequence independent and may involve p53 binding to components of the basal transcription machinery, such as the TATA-binding protein [e.g. Sero, E. et al, Proc. Natl. Acad. Sci. U.S.A. 89, 12028-12032 (1992), Truant, R. et al, J. Biol. Chem. 268, 2284-2287 (1993), Liu, X. et al, Mol. Cell. Biol. 13, 3291-3300 (1993)]. In contrast, transactivation by p53 is sequence dependent and correlates with its binding to specific DNA sequences such as the recently reported consensus-binding site [El-Deiry, W. S. et al, Nature Genetics 1, 45-49 (1992), Funk, W. D. et al, Mol. Cell. Biol. 12, 2866-2871 (1992)]. p53 can efficiently activate transcription from promoters bearing such sites, both in vivo and in vitro [e.g. Kley, N. et al, Nucleic Acids Res. 20, 4083-4087 (1992), Seto, E. et al, Proc. Natl. Acad. Sci. U.S.A. 89, 12028-12032 (1992), Weintraub, H. et al, Proc. Natl. Acad. Sci. U.S.A. 88, 4570-4574 (1991), Kern, S. E. et al, Science 256, 827-830 (1992), Farmer, G. et al, Nature 358, 83-86 (1992), Zambetti, G. P. et al, Genes & Development 6, 1143-1152 (1992)]. Most oncogenic mutants of p53 have lost both the transcription suppression and sequence specific transactivation properties displayed by wild-type p53.
The strong correlation between the ability of p53 to activate transcription in a sequence specific manner and its ability to suppress cell growth or induce apoptosis [Vogelstein, B. and Kinzler, K. W., Cell 70, 523-526 (1992), Yonish-Rouach, E. et al, Nature 352, 345-347 (1991), Lowe, S. W. et al, Nature 362, 847-849 (1993), Clark, A. R. et al, Nature 362, 849-852 (1993), Shaw, P. et al, Proc. Natl. Acad. Sci. U.S.A. 89, 4495-4499 (1992)], suggests that p53-induced genes may play a critical role in mediating the function of p53 as a tumor suppressor. A few endogenous genes have been characterized to be induced by p53. These include the mdm-2 and its human homolog hdm-2 [Wu, X. et al, Genes & Development 7, 1126-1132 (1993)], GADD45 [Kastan, M. B. et al, Cell 71, 587-597 (1992)], and WAF1/CIP1/p21 [El-Deiry, W. S., et al, Cell 75, 817-825 (1993)] genes. hdm-2 has been suggested to act as a negative feedback regulator of p53, and in this respect would function as an oncogene [Wu, X. et al, Genes & Development 7, 1126-1132 (1993), Zambetti, G. and Levine, A. J., FASEB J. 7, 855-865 (1993)]. This is consistent with amplification of the hdm-2 gene being associated with human cancers [Oliner, J. D. et al, Nature 358, 80-83 (1992)]. Both WAF1/CIP1/p21, an inhibitor of cyclin-dependent kinases [Harper, J. W. et al, Cell 75, 805-816 (1993), Xiong, Y. et al, Nature 366, 701-704 (1993)], and gadd45 [Zhan, Q. et al, Mol. Cell. Biol. 14, 2361-2371 (1994)] have so far been shown to inhibit growth of tumor cells in culture [El-Deiry, W. S. et al, Cell 75, 817-825 (1993)].